MAINTAINERS: Fix documentation file name for PHY Library

MAINTAINERS still pointed to phy.txt after moving this file into the rst
format, fix this.

Reported-by: Joe Perches <joe@perches.com>
Fixes: 25fe02d0 ("Documentation: net: phy: switch documentation to rst format")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>